1. Field of the Invention
The present invention relates to a first grid circuit for a monitor display, and more particularly to a first grid muting circuit wherein a first grid circuit included in a cathode ray tube (CRT) is smoothly muted during mode conversion or power-on state of a monitor to prevent distorted display of raster, thereby providing clear picture on the CRT.
2. Description of the Prior Art
Generally, first grid muting circuits serve in such a manner that thermoelectrons are emitted to collide against a phosphor screen with thin electron beams to arbitrarily control the quantity of electrons colliding against the phosphor screen by means of an electron gun. The electron gun consists of a cylindrical cathode, grid and anode, in which a first grid in the grid is an inhibit grid for controlling the quantity of the electrons emitted from the cathode to control the variations of luminescence or brightness.
FIG. 1 is a block diagram showing a general muting circuit according to a conventional technique.
The muting circuit includes a muting detection portion 1 for receiving horizontal and vertical sync signals to output a muting signal upon mode conversion or power-ion state, a video muting control portion 2 connected to the muting detection portion 1 for receiving the muting signal therefrom to output a control signal, and a video contrast portion 3 connected to the video muting control portion 2 for receiving the control signal therefrom to control a video signal, thereby varying a luma signal.
In the conventional muting circuit constructed as above, once the horizontal and vertical sync signal among display signals from a computer are supplied to the muting detection portion 1, the muting detection portion 1 detects the input signal at the initial state of the sync signals (i.e., the time of power on), or at the changing state of the polarity of the sync signals (i.e., the time of mode conversion), and then supplies the muting signal to the video muting control portion 2. The video muting control portion 2 cuts off the video contrast portion 3 connected to the output terminal thereof, so that the control is canceled to mute the video signal.
At this time, the video contrast portion 3 is formed to increase or decrease the amplification of the video signal of the computer.
However, since the muting circuit is formed to control the video signal, raster distortion occurs on the picture when power of the monitor is turned on or the mode is converted at the time of reviving the back raster (i.e., at the time of shifting the CRT from a cut-off state to an active state).